As known, the most common ski mountaineering boots substantially consist of a shell made of rigid plastic material which is shaped so as to accommodate the user's foot, and is provided on the bottom with a front sole and a rear heel, usually provided with a lugged profile and made of a non-slip elastomeric material; with a cuff made of a rigid plastic material, which is C-shaped so as to envelop the user's ankle from behind, and is hinged to the upper part of the shell so as to oscillate about a transversal reference axis substantially coinciding with the articulation axis of the ankle; with an inner shoe made of soft, heat-insulating material, which is removably inserted into the shell and the cuff, and is shaped so as to envelop and protect both the foot and the lower part of the user's leg; and with a series of manually-operated closing hooks, which are appropriately distributed on the shell and on the cuff, and are structured so as to tighten the shell and the cuff in order to immobilize the user's leg inside the shoe.
Furthermore, the shell of the ski mountaineering boots is provided on the front with a small, substantially duck-billed projecting appendix, which protrudes from the nose-shaped tip of the shell remaining locally substantially coplanar with the front sole, and is structured so as to be coupled in a rigid, stable, although easily releasable manner, with the toepiece of the ski mountaineering binding device which, in turn, is rigidly fixed onto the central part of the downhill ski.
The ski mountaineering binding device instead consists of a toepiece and a heelpiece, which are rigidly and stably fixed to the back of the downhill ski, at a predetermined distance from each other, and are structured so as to alternatively and as desired:                lock the shell of the ski boot onto the back of the ski, thus preventing any relative movement between the two elements; or        lock the shell of the ski boot onto the back of the ski thus allowing the boot to freely oscillate/pivot with respect to the ski about a transversal rotation axis arranged horizontally and roughly positioned at the duck-billed appendix of the shell.        
Obviously, the rotation axis of the ski boot is perpendicular to the rotation axis of the downhill ski, i.e. is oriented so as to be locally substantially perpendicular both to the middle plane of the ski and to the middle plane of the ski boot.
In particular, the toepiece is usually provided with a gripper-like clamping member, which is structured so as to clamp and stably retain the projecting duck-billed appendix of the shell, while allowing the shell to freely oscillate/pivot with respect to the ski underneath about the rotation axis of the boot. The heelpiece of the binding device, instead, is structured so as to selectively hook and lock the rear part of the shell, so as to selectively prevent the boot from rotating by pivoting on the toepiece and moving the heel away from the back of the ski.
In ski mountaineering binding devices currently on the market, shifting from the configuration which completely locks the shell onto the back of the ski to the configuration which allows the ski mountaineering boot from freely oscillating/pivoting on the back of the ski by pivoting on the toepiece always requires the complete unlocking of the boot from the ski and the reconfiguration of the binding device as a function of the new use.
Unfortunately, hooking the duck-billed appendix of the shell to the toepiece of the ski mountaineering binding device is a relatively laborious operation, which may create some problems to the least expert skiers, especially when operating on fresh snow or however in bad weather conditions.